The present invention relates generally to a radio communication apparatus using an adaptive antenna, which is applied to a mobile radio communication system, etc., and more particularly to a transmission beam control apparatus for desirably controlling antenna directivity at the time of transmission.
In land mobile communications, there will frequently occur degradation in received signal level due to fading, or a signal distortion due to co-channel interference (CCI) or intersymbol interference (ISI).
In such a severe environment for signal propagation, in order to exactly extract a desired wave, it is effective to use an adaptive antenna which suitably controls antenna directivity. Adaptive arrays are well-known as an example of adaptive antennas. Adaptive arrays possibility suppress interface signals by pointing nulls to interfacing stations.
In land mobile communications, however, the size of a terminal station needs to be reduced for portability. In general cases, adaptive arrays are applied to a base station because of having plural antennas.
Adaptive arrays are array antennas which synthesize received signals from a plurality of antennas by controlling the phases and amplitudes of the signals. Even when high-level interference waves are present, a beam is turned to a direction from which a desired wave is incident and nulls (a point with no gain) are turned to directions from which interference waves are incident. Thereby, a reception SIR (a desired wave to interference wave ratio) can be increased to a maximum.
The operation of controlling and synthesizing the phases and amplitudes of received signals is equivalent to an operation wherein received signals from plural antennas #1, #2, . . . , #N are subjected to complex weighting, as shown in FIG. 1, by means of complex multipliers 1-1, 1-2, . . . , 1-N, which multiply N received signals by reception weight vectors calculated by a reception weight vector calculation section 2, and the resultant signals are synthesized by an adder 3. In this case, an output y (“adaptive array output”) from the adder 3 is given byy=wTx  (1)In this equation, w is a complex weight vector (hereinafter referred to as “reception weight vector”) applied to the received signal from each antenna, and x is a complex received signal vector from each antenna, w and x being expressed byw=(w1, w2, . . . , wj, . . . , wN)T  (2)x=(x1, x2, . . . , xj, . . . , xN)T  (3)wherein T denotes transposition of matrix.
The reception weight vector w is controlled so that the adaptive array output y may satisfy a predetermined criterion. According to criteria, for example, an average square error between the adaptive array output y and an ideal signal sequence is reduced to a minimum, or a signal power of the adaptive array output y is reduced to a minimum under a constraint on the direction of arrival (DOA) of a desired wave. In this way, with respect to the reception in the base station (that is uplink), the received signals from plural antennas are weighted and consequently a distortionless signal can be obtained.
The above description is directed to the case where a omni-directional antenna is used as antenna, and this type of signal processing is called “element space processing.” On the other hand, there is known “beam space processing” wherein a plurality of beams with different directions of radiation are formed in advance and received signals obtained by the beams are subjected to an adaptive array processing.
If a beam space adaptive array is used, a pre-processing beam generator needs to be additionally provided. However, since a signal output with a high SNR (signal-to-noise ratio), combined with a beam gain, is obtained, a stable adaptive array processing can be expected by selecting appropriate beams. Moreover, since the number of branches input to the adaptive array can be reduced, the amount of arithmetic operations for signal processing can be reduced accordingly.
This feature is described, for example, in document [1] (Chiba, Nakajo, Fujise, “BEAM SPACE CMA ADAPTIVE ARRAY ANTENNA”, IEICE Transaction of the Communications, B-II, vol. J77-B-II, no. 3, pp. 130–138, March 1994).
In general, a beam space adaptive array generates spatially orthogonal beams. However, as non-orthogonal beams, an adaptive antenna using directional antennas overlapping between adjacent beams has been studied.
For example, document [2] (Jpn. Pat. Appln. KOKAI Publication No. 10-256821 (Matsuoka, et al.)) proposes an adaptive antenna capable of efficiently combining delay wave energy by performing not only a space domain process using an adaptive array antenna but also a time domain process using path diversity.
On the other hand, many downlink beam forming methods to synthesis optimal transmission beam pattern using an array antenna has been studied. For example, in TDD (Time Division Duplex) system wherein transmission/reception is periodically switched by time division, since the same frequency is used in the transmission/reception, it can be regarded that the propagation channel responses of transmission and reception are substantially equal.
Accordingly, as shown in document [3] (Tomisato, Matsumoto, “EFFECT OF ADAPTIVE TRANSMISSION ARRAY IN TDD MOBILE COMMUNICATION SYSTEM”, 1997-IEICE Spring conference, B-5-87, March 1997), the reception SIR at the terminal station can be improved by using the same weight vector for transmission/reception, i.e., by forming the same antenna pattern in transmission as is obtained at the time of reception.
However, as in the case of FDD (Frequency Division Duplex) where different frequencies are used for transmission and reception, the correlation in propagation channel response between uplink and downlink is small. Thus, even if a transmission weight vector that is equal to a reception weight vector is used, optimal reception at the terminal station is not always ensured (e.g. see document [4] (J. Litva, T. K.-Y. Lo, “Digital Beamforming in Wireless Communications,” Artech House Publishers, pp. 182–183, 1996).
As stated above, although the propagation channel response differs between uplink and downlink, there is reversibility between uplink and downlink with respect to the direction of arrival of radio waves. Specifically, except for a case where the speed of movement of the terminal station is excessively high, the reception SIR at the terminal station can be increased to a maximum by estimating DOA of reception ratio waves at the base station and setting the beam and null in that direction.
For the purpose of such transmission beam pattern control, the estimation of the DOA is indispensable. As a signal processing for the estimation of the DOA, there is known a MUSIC (MUltiple SIgnal Classification) algorithm, etc. are known.
However, a great amount of calculations is required for a high-resolution DOA estimation algorithm represented by MUSIC. This algorithm is not suitable in a case of estimating the DOA which varies from time to time depending on the movement of the terminal station or a variation in environment.
Even if numerous arithmetic operations are performed to precisely estimate the DOA, and the weighting for directivity is carried out to set the null in the estimated DOA, the direction of the null may deviate due to defective calibration of the transmission circuit. Furthermore, the effect of this technique may deteriorate due to the angle spread by reflection/dispersion near the terminal station in the actual propagation path. As a result, the average reception SIR at the terminal station may deteriorate.
Besides, if the number of incoming waves is greater than the number of antennas in the multi-path environment, it is difficult to estimate the DOA by MUSIC.
The present invention has been made to solve the above problems, and its object is to provide a radio communication apparatus which is applicable to a system using different frequencies for uplink and downlink and can easily estimate a DOA of radio waves and enhance an average reception SIR at an opposing-side station.